Ink-jet printing is frequently categorized as a nonimpact or impactless printing technology in which a uniform train of ink droplets is generated by a Rayleigh instability. These ink droplets are then directed onto paper to produce text and graphics.
Numerous variations of ink-jet printers exist although only two modes of operation appear to have been widely studied.
The first mode of operation is known as the drop-on-demand system in which droplets of ink are generated as needed and are ejected from 20-80 micrometer orifices producing a stream of ink droplets with a velocity of .about.3 m sec.sup.-1. In the drop-on-demand system ink droplets are produced either by thermal bubble formation in which a vapor bubble is formed which forces an ink droplet through the orifice or by a piezoelectric crystal vibrating at high frequency causing the ink passing through the orifice to be broken into minute droplets equal in number to the crystal vibrations. The droplets are assigned the correct trajectory by the writing head in order to produce characters on paper.
The second mode of operation is known as synchronous ink-jet printing in which ink droplets are produced continuously. This is achieved by pressurizing the ultrasonically attenuated jet to .about.3.times.10.sup.5 Pa which produces a stream of droplets (10.sup.6 per second) with a velocity of .about.20 m sec.sup.-1. The ink drops used to generate characters are inductively charged and deflected in a high voltage electric field to a specific position on paper. The uncharged ink droplets pass undeflected through the electric field to be caught in a gutter and recirculated through the fluidic circuit.
Inks which are suitable for use in an ink jet printing system should display a consistent drop breakup length, drop velocity and, for synchronous ink jet printing, drop charge under set operating conditions. Conventional ink jet inks are complex multicomponent systems containing dye(s), polymers, solubilizing agents, chelating agents and biocides.
Several drawbacks plague the use of dye-containing, water-based inks. Dyes suffer a limited color selection and gamut when compared to pigments. This results in limited color reproduction. Furthermore, dyes are not as lightfast and waterfast as pigments. Dyes also tend to wick and bleed into uncoated and rough papers. The degree of spreading depends upon the paper used. Thus, the range of papers which can be used with dye-based inks is limited. In addition there is a loss in image resolution due to feathering and intercolor bleeding of image characters on the paper.
Because of the limitations of the water-soluble dye approach, researchers have worked with pigment-based inks. Although pigment-based inks are more lightfast and waterfast and can be used with a wide range of papers, it is still very difficult to obtain particles small enough which do not settle and clog the orifice of the printing system. The orifice must remain open for a wide range of operating conditions, e.g., temperature and humidity.
Conventional processes for preparing pigment-based ink jet inks include blending ink jet materials together and then mixing and grinding with a known dispersing device such as a ball mill, homomixer, sand mill or roll mill. Japanese Patent Application Publication No. 61-57,669 which was laid open on Mar. 24, 1986 and Japanese Application Publication No. 61-168,677 which was laid open on July 30, 1986, describe the preparation of pigment-based inks using these conventional techniques.
U.S. Pat. No. 4,597,794 describes a process wherein the pigmented ink jet ink is prepared by dispersing fine particles of pigments in an aqueous dispersion medium containing polymer having both a hydrophilic and hydrophobic construction portion. The components were prepared using the standard processes described above.
One of the problems with the conventional methods used to prepare pigmented ink jet inks is that they are very time consuming. It is believed that pigment dispersions prepared by standard milling processes, generally, do not possess sufficiently small particle size and are not sufficiently stable. Accordingly, when incorporated into a printing system, the ink particles tend to agglomerate resulting in the clogging of the small nozzles present in the printing heads of the ink jet devices. There is still a need for a process which enables the quick preparation of a pigmented ink jet ink wherein the ink contains a substantially reduced degree of agglomeration so that the ink does not clog the nozzles of the ink jet printing system and the resulting deflocculated ink does not re-agglomerate during storage or jetting.